A Web-based System for Handling Multidimensional ... - CiteSeerX

A Web-based System for Handling Multidimensional Information through MXML? Manolis Gergatsoulis1, Yannis Stavrakas1 2, Dimitris Karteris1, Athina Mouzaki1, and Dimitris Sterpis1 ;

Institute of Informatics & Telecommunications, National Centre for Scienti c Research (N.C.S.R.) `Demokritos', 153 10 Aghia Paraskevi Attikis, Greece. 2 Knowledge & Database Systems Laboratory National Technical University of Athens (NTUA), 157 73, Athens, Greece. 1

fmanolis,ystavr,[email protected] [email protected]

Abstract. In this paper we address an issue common in the frame of

WWW, namely information entities that present dierent facets under dierent contexts (or worlds). Handling such multifacet or multidimensional entities requires a multidimensional paradigm for Web data, which consists of representation, manipulation and presentation issues. For representing multidimensional data we employ Multidimensional XML, a markup language that incorporates dimensions in XML. We discuss the presentation of multidimensional data through multidimensional XSL stylesheets. We describe the design of a system that implements the basic functionality of the multidimensional paradigm, and demonstrates how a user can interact with a multidimensional document and view dierent variants of the document under dierent worlds. Keywords: Multidimensional XML/XSL, Web Databases, Multidimensional Languages.

1 Introduction The wide acceptance of WWW was due, to a great extend, to the simplicity of its protocol (HTTP) and language (HTML). However, the need for more sophisticated functionality led to secure protocols (HTTPS), exible languages (XML) [5,7] and ambitious visions for the future of the Web (Semantic Web [4]). Viewing the Web as a large database, a number of query languages and data models, such as semistructured data [1,15], have been proposed. However, those models fall short when it comes to representing multidimensional information; that is, information that presents dierent facets under dierent contexts. Actually, there are many cases where variants of the same entity do exist. As a ?

This work has been partially supported by the Greek General Secretariat of Research and Technology under the project \Executable Intensional Languages and Intelligent Applications in Multimedia, Hypermedia and Virtual Reality" of  ENE
'99.

simple example imagine a report that needs to be represented at various degrees of detail and in various languages. A solution would be to create a dierent document for every possible combination. Such an approach is certainly not practical, since it involves excessive duplication of information. What is more, the dierent variants are not associated as being parts of the same entity. The problem of varying entities is very common in the frame of the WWW, where information providers cannot assume too much about the background context of the information consumers. For those reasons, models and languages suitable to represent and exchange multidimensional data over the Web are needed. Ideas on how this problem can be tackled are given in [14,13], where a formalism called Multidimensional XML (MXML) is presented. MXML was in uenced by Intensional HTML (IHTML) [17,6,16]. IHTML is a Web authoring language, based on and extending ideas proposed for a software versioning system in [12], that allows a single Web page to have dierent variants and to dynamically adapt itself to a given context. The main dierence between IHTML and MXML is a projection of the dierence between HTML and XML, that is, focusing on encoding structure rather than on presentation. In this paper, we propose a multidimensional paradigm for representing and viewing context-dependent Web data. We present a comprehensive example that demonstrates the syntax and relationships of the various multidimensional components in the paradigm. We describe the architecture and design of a system that implements the basic functionality of the proposed paradigm, and discuss some implementation issues. Finally, we conclude the paper with directions for future work.

2 A Multidimensional Paradigm The problem of Web information entities that may assume dierent facets under dierent contexts (or worlds) leads to a new paradigm for representing and viewing such entities. We refer to the new paradigm as multidimensional paradigm, and in this section we examine its various aspects.

2.1 The Multidimensional Approach A widely adopted way to encode, represent, and exchange information in the frame of WWW is through eXtensible Markup Language (XML in short) [5,7]. XML is exible enough to adapt to dierent domains, and capable of handling the irregularities often exhibited by Web data. XML does not address the issue of how to present information; a solution to this is oered by eXtensible Stylesheet Language (XSL in short) [2,8]. XSL can be seen as a document containing instructions on how to present information in XML documents. It is important to note that XSL stylesheets are also XML documents. An XSL stylesheet can be applied to a speci c XML document, and the result can be displayed by a Web browser. Actually, a number 2

XSL XML

w1,1

XML w1

XML w1,k1 ... w2,1

w2 MXML

XSL XML

XML

XML

...

w2,k2

. . .

MXSL

. . . wn,1 ...

wn

XSL XML

XML

wn,kn

XML

Fig. 1. Multidimensional XML, multidimensional XSL, and possible worlds. of XSL documents can be applied to the same XML document, resulting in dierent ways to view that XML document, or parts of it. A multidimensional approach would allow a single document to have a number of variants, each holding under a speci c world. A world is de ned by giving values to a number of parameters called dimensions. We assume that information in such a document is encoded in a suitable markup language called multidimensional XML (MXML in short). Once a world is speci ed, such an MXML document can be reduced to a conventional XML document that constitutes the holding facet under that world. This world-associated decomposition of an MXML document to a number of conventional XML variants is depicted in Figure 1, where the possible worlds for the MXML document are denoted as w1; w2.. . w . Since XSL stylesheets are XML documents, the same principles hold for XSL stylesheets as well. A multidimensional XSL stylesheet (MXSL in short) encodes a set of conventional XSL stylesheet, each being the facet of the MXSL under a speci c world. Like conventional XSLs, an MXSL must be associated with an XML or an MXML document. For each possible world, the holding XSL is applied to the holding XML to give the view of the document under that world. The relation between MXML and MXSL is given pictorially in Figure 1. Note that the possible worlds for an MXSL may not be identical with those of the corresponding MXML. A number of dimensions in an MXSL may relate to the de nition of alternative presentations for the same XML document. Besides, some dimensions of an MXML may also be used in the corresponding MXSL to establish a correspondence between the holding variants of MXML and MXSL. n

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2.2 Multidimensional XML

For supporting the multidimensional approach described above, we propose Multidimensional XML (MXML for short) [14,13,10], a markup language that in-

corporates in an elegant way multidimensional capabilities in XML. The notion of world is fundamental in MXML. A world represents an environment under which data in a multidimensional document obtain a substance. A world is determined by assigning values to a set S of dimensions. For each dimension d there exists a domain D over which d ranges. A world W is de ned to be a set which, for each dimension d 2 S , contains a pair (d; u), where u 2 D . In an MXML document, dimensions may be applied to elements and attributes. An element whose content depends on one or more dimensions is called multidimensional element, while an attribute whose value depends on one or more dimensions is called multidimensional attribute. An MXML document uses context speci ers which are syntactic constructs that specify sets of worlds. Context speci ers qualify the variants of multidimensional elements or attributes, relating each variant to the set of worlds under which the variant becomes the holding one for the corresponding multidimensional entity. In MXML syntax, a multidimensional element has the form: d

d

[context_specifier_1] element_content_1 [/] . . . [context_specifier_N] element_content_N [/]

A multidimensional element is denoted by preceding the element name with the special symbol \@". A multidimensional element encloses one or more context elements that constitute facets of that multidimensional element, holding under speci c worlds speci ed by the corresponding context speci er. Context elements have the same form as conventional XML elements. All context elements belonging to a multidimensional element have the same name which is the name of the multidimensional element. In MXML, context elements of the same multidimensional element may have dierent value or structure. To declare a multidimensional attribute we use the following syntax: attribute_name = [context_specifier_1] attribute_value_1 [/] . . . [context_specifier_n] attribute_value_n [/]

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A multidimensional attribute is expressed as an attribute whose value is a set of context - value pairs. Each one of those context-associated values becomes the holding value of the attribute under the corresponding context. Therefore, under dierent worlds, an attribute may evaluate to dierent values depending on its context speci ers. A context speci er is of the form: dimension 1 specifier, ..., dimension m specifier

where dimension i specifier, for i = 1 to m is a dimension speci er of the form: dimension name

specifier operator

dimension value expression

A speci er operator is one of =, ! =, in, not in. If the speci er operator is either = or ! =, the dimension value expression consists of a single dimension value. Otherwise, if the speci er operator is either in or not in, the dimension value expression is a set of values of the form fvalue1; : : :; valuekg, with k  1. In both multidimensional attributes and elements, a context speci er may also be the reserved word \default". The default context speci er represents all worlds not covered by other context speci ers of the same multidimensional entity. The empty context speci er [ ] is called universal context speci er, and represents the set of all possible worlds. A multidimensional DTD (MDTD in short) has been proposed in [10] for de ning constraints on the structure of MXML documents. An MDTD allows to specify dimensions and their respective domains. Moreover, it is possible to impose dierent constraints under dierent contexts to an element structure. In this way, an MDTD can specify that an element has dierent structure under dierent sets of worlds. A graph data model for MXML has also been proposed in [10] that takes into acount contexts when representing MXML data.

2.3 Reducing MXML to XML An important point concerning the context speci ers of a multidimensional entity is that they must be mutually exclusive, in other words, they must specify disjoint sets of worlds. This property of multidimensional entities makes it possible, given a speci c world, to safely reduce an MXML document to an XML document holding under that world. Informally, the reduction of an MXML document D to an XML document D holding under the world w proceeds as follows: Beginning from the document root to the leaf elements, each multidimensional element E is replaced by its context element E , which is the holding facet of E under the world w. If there is no such context element, then E along with its subelements is removed entirely. A multidimensional attribute A is transformed into a conventional attribute A whose name is the same as A and whose value is the holding one under w. If no such value exists then the attribute is removed entirely. w

w

w

5

Notice that the above process can be generalized for producing a facet that holds under a set of more than one worlds. In the general case, that facet is an MXML document which is a pruned version of the original MXML document.

2.4 A Comprehensive Example As an example of the multidimensional paradigm, consider information about a book which exists in two dierent editions, an English and a Greek one. In Example 1, the element book has six subelements. The isbn and publisher are multidimensional elements and depend on the dimension edition. The elements title and authors remain the same under every possible world. The element price is a multidimensional element whose value depends on the dimensions edition and customer type. Note that the element translator has substance only under the worlds where edition has the value greek. Example 1. Multidimensional Information about a book encoded in MXML. [edition = greek] 0-13-110370-9 [/] [edition = english] 0-13-110362-8 [/] The C programming language Brian W. Kernighan Dennis M. Ritchie [edition = english] Prentice Hall [/] [edition = greek] Klidarithmos [/] [edition = greek] Thomas Moraitis [/] [edition=english,customer_type=individual]13.000[/] [edition=english,customer_type=library]10.000[/] [edition=english,customer_type=student]11.700[/] [edition=greek,customer_type=individual]5.000[/] [edition=greek,customer_type=library]3.000[/] [edition=greek,customer_type=student]4.500[/]

The MXML in Example 2 is an MXSL stylesheet for the MXML document in Example 1. This MXSL speci es how the various facets of the corresponding MXML in Example 1 are to be presented. The ISBN is shown only if the request has been made by a library, while title and authors are shown if the potential client is an individual or a student. Note 6

how the wrapper element, which is de ned in XSL, can also be used elegantly in MXSL to exclude the translator in case the request concerns the original language edition of the book. Finally, publisher and price are displayed in any case. Notice that an MXSL may contain multidimensional versions of the elements and attributes de ned within the frame of conventional XSL. There is no constraint on which dimensions participate in the MXSL context speci ers; they may also occur in the corresponding MXML document, as is the case of dimensions customer type and edition in Example 2, or they can be dierent, as is the case of dimension size in the same example. Example 2. An MXSL multidimensional stylesheet for Example 1. [customer_type = library] ISBN: , [/] [customer_type in {individual, student}] Title: , Authors: , [edition=greek] Translator: , [/] [/] Publisher: , Price:

For the world w = f(edition, greek), (customer type, student)g, the MXML document in Example 1 is reduced to the conventional XML document in Example 3. 7

Example 3. 0-13-110370-9 The C programming language Brian W. Kernighan Dennis M. Ritchie Klidarithmos Thomas Moraitis 4.500

For the world w = f(edition, greek), (customer type, student), (size, large)g, the MXSL stylesheet in Example 2 is reduced to the XSL stylesheet in Example 4. 0

Example 4. Title: , Authors: , Translator: , Publisher: , Price:

The result of applying the XSL stylesheet of Example 4 to the XML document of Example 3 looks like the following:

Book

Title: The C programminglanguage, Authors: Brian W. Kernighan, Dennis M. Ritchie, Translator: Thomas Moraitis, Publisher: Klidarithmos, Price: 4.500 Finally, the analogous result for the world w = f(edition, english), (customer type, library), (size, normal)g would look like this: 0

Book

ISBN: 0-13-110362-8, Publisher: Prentice Hall, Price: 10.000 8

3 System Architecture In this section we describe a prototype system that demonstrates the basic principles of the multidimensional paradigm we introduced in the previous sections. The implemented system is called MXML Web Server and is a Web server capable of handling multidimensional data encoded in MXML/MXSL. In a typical scenario, the user requests an MXML document through a conventional Web browser, and is prompted to select values for each of the dimensions associated with a requested document. After a world has been speci ed by the user, the server sends the corresponding XML/XSL facet to be displayed by the browser. The user can change the values of dimensions and observe how dierent worlds are associated to dierent variants of the same multidimensional document. An important point is that, in the multidimensional paradigm, the management of dimensions and the reduction of MXML/MXSL can take place at the server, the client, or both. Our system implements this functionality at the server side mainly for reasons of compatibility with existing Web browsers. In the general case, however, some of the dimensions can be considered at server side, to eliminate irrelevant data and reduce the size of the response, while the rest of the dimensions could be handled at client side, for reasons of privacy or for minimizing the number of subsequent requests. This exibility could be useful and further explored in the frame of applying MXML to domains such as electronic commerce and user modeling.

3.1 Extending URLs URL (Uniform Resource Locator) [3] is the standard way to specify a resource available on the Internet. We extend the syntax of URL in order to enable it to handle multidimensional information. The extended URL has the form: http://:/?

The only dierence from conventional URL is that the token has been added, in order to incorporate dimensions. In the following example we show an extended URL that represents a request for the variant of the document named books.mxml, where edition is english and customer type is student. http://myserver/books.mxml[edition=english,customer_type=student]

3.2 Design and Operation The system comprises the software modules illustrated in Figure 2. These modules are: Request Analyzer, MXML Request Decomposer, View Extractor, MXML Response Composer, and Conventional Web Server. In the following paragraphs we shall brie y describe the functionality of each of those modules, as well as their submodules, and discuss their role in the system operation. 9

Client

Client

Client Internet

client request

MXML Web Server

MXML Request Decomposer MXML request

final response (XML,HTML,TEXT, ...)

Request Analyzer request type

MXML user file context identifier

normal request MXML Response Composer

View Extractor (M)XML

MXML file

response Conventional Web Server

MXSL file

(M)XSL

status information

file repository

XML,HTML,TEXT, ... files

Fig. 2. MXML Web Server architecture.

Request Analyzer: The Request Analyzer is responsible for determining the

type of client requests. There are two types of requests; normal requests, which refer to HTML, XML, text les etc., and MXML-requests, which refer to MXML les. If the request is of normal type, then the Conventional Web Server handles it. Otherwise, if the request involves an MXML document, then the MXML Request Decomposer is invoked to serve the request.

MXML Request Decomposer: This module decomposes the MXML re-

quest into sections. The three sections that comprise the MXML request are: the MXML File Identi er, the User Context and the Request Type. The rst two sections are sent to the View Extractor module, while the third is sent directly to the MXML Response Composer.

View Extractor: This module combines the information provided by the MXML Request Decomposer with the corresponding MXML or MXSL les in order to serve the request. The View Extractor consists of two sub-modules called MXML Parser and MXML Specializer, as it is depicted in Figure 3(a). { MXML Parser: The MXML Parser uses the requested MXML File Identi er, which is provided by the MXML Request Decomposer, to access the 10

Request Analyzer

file repository

View MXML file Extractor

MXSL file

requested MXML file identifier

normal request Conventional Web Server

MXML Parser Response Composer MXML parse tree

MXSL parse tree

Response Composer

response user context Response Sender

MXML Specializer

response to MXML request

final response (XML,HTML,TEXT, ...)

context specific context specific status (M)XML (M)XSL information

(a)

(b)

Fig.3. (a) View Extractor's sub-modules, (b) Conventional Web Server's sub-modules. corresponding MXML or MXSL le from the File Repository. Its role is to parse the le and generate the corresponding MXML or MXSL parse tree. { MXML Specializer: The MXML Specializer uses the parse tree that is generated by the MXML Parser, and the User Context, which is provided by the MXML Request Decomposer, to generate a context speci c (M)XML or (M)XSL le. This is done by reducing the original MXML/MXSL le to an (M)XML/(M)XSL le that holds under the speci ed context. The type of the produced le is passed to MXML Response Composer through the status information.

MXML Response Composer: The MXML Response Composer constructs

the response and sends it to the Response Sender, which is responsible for dispatching the response to the client.

Conventional Web Server: The Conventional Web Server implements some

of the essential features of a conventional Web server. It consists of two submodules, shown in Figure 3(b). In case the type of request is normal, the response is constructed by the Response Composer submodule that is part of the Conventional Web Server, whereas in case the type of request is MXML-request the response is produced in MXML Response Composer module and is passed to the Response Sender submodule of the Conventional Web Server. 11

{ Response Composer: The Response Composer analyzes the normal re-

quest, and constructs the response that is sent to the client. As it is depicted in Figure 3(b), this module is engaged in a client request service procedure only if this request is of type normal. { Response Sender: The Response Sender is responsible for sending the response to the client. As stated above, the response originates either from the Response Composer, or from the MXML Response Composer.

4 Implementation The system components described in the previous section are in fact implemented as two separate programs. The MXML Web Server (except View Extractor) was developed in Java in order to take advantage of the powerful features that this language oers for network programming. The View Extractor has been implemented in ANSI C.

Fig. 4. A screenshot showing a reply from MXML Web Server. 12

Figure 4 is a screenshot of the reply of MXML Web Server to the request: http://143.233.3.195:1111/menu.mxml[season=spring,

.

detail=high,language=English]

The context speci er in this extended URL is composed dynamically by JavaScript code, based on the values selected by the user for the dimension drop-down lists, in the upper left dimension frame. Note that for every dimension there exists a value \ANY" in the corresponding drop-down list. The meaning of \ANY" is that no value is speci ed for that dimension. Consequently, if a context speci er contains one or more \ANY", it speci es more than one worlds. If it does not contain \ANY" at all, it speci es exactly one world. The upper-right action frame allows the user to view the MXML source code, and the MXSL source code. If the selected context speci es exactly one world (does not contain \ANY") then the user may also see the XML and XSL sources of the corresponding facets under that world, plus the nal output document. The source code and the nal document are displayed in the lower output frame, which in the case of Figure 4 shows a variant of a multidimensional menu of a restaurant corresponding to the world w = f(season, spring), (detail, high), (language, English)g. The implemented system can be reached at the URL:



http://www.iit.demokritos.gr/ mxml

5 Conclusions and Future Work In this paper we proposed a paradigm for handling multidimensional data in the frame of the Web. We proposed the use of MXML to represent multidimensional information, and showed how multidimensional stylesheets can be expressed in MXSL. Through a comprehensive example we demonstrated the reduction of MXML/MXSL to conventional XML/XSL under a speci c world. Finally, we presented a system that implements the basic functionality of the proposed paradigm, and discussed some implementation issues. Our future plans include the investigation on possible applications of MXML in diverse elds, such as electronic commerce and digital libraries. We also consider the representation of time dependent data through MXML [11], the representation of cartographic and GIS information where a possible dimension is scale, and applications where user pro ling information plays an important role for delivering the right data. Since MXML is primarily considered as a data model and data exchange format, an interesting direction is how to eciently store and retrieve MXML data. Approaches used for XML, that employs RDBMS for storing XML fragments [9], seem promising for adaptation to MXML. 13

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